Electron beam testing techniques are becoming established as a means of measuring waveforms within operating integrated circuits (ICs). One area of application of electron beam testing technology is to the measurement of very high speed signals within ICs. One reason for this is that IC technology has so advanced that signals with many GHz bandwidth can be generated on a circuit. However, the electrical properties of IC packages make it very difficult to take this signal outside the chip. It is thus possible to design circuits which operate at high speeds internally but only communicate to the outside world at a much lower speed. Nevertheless, it is essential to have a means of measuring the ultra high frequency internal waveforms of a circuit in order to verify correct internal operation and to debug circuits which function incorrectly.
Traditionally, mechanical probes have been used to measure waveforms within circuits. However, as with the packages themselves, the electrical characteristics of the probes are such that very high speed signals cannot be measured. Electron beams have also been used to measure waveforms within circuits. The bandwidth of electron beam detection systems is fundamentally limited to relatively slow speeds (absolute maximum of a few MHz). Therefore, to make high speed measurements stroboscopic techniques must be used. This is generally achieved by pulsing the electron beam in synchronism with the circuit and integrating the collected signal.
With this last-mentioned technique, temporal resolution is limited by the speed with which the beam pulses can be switched and by the accuracy with which the pulses can be made to be incident at the same place in a waveform. In conventional beam pulsing systems, the electron beam is deflected electrostatically using a pulse generator so that, for a short period, the beam passes on down the axis but for the period outside the pulse time the beam is deflected on to an aperture where it is stopped. This system is limited by the speed of the pulse generator and the electron transit time through the blanking plates.
Higher speed signals are made possible in one known arrangement by scanning the electron beam across an aperture. A ramp signal is applied to the blanking plates but the beam is only positioned over the aperture for input voltages corresponding to a small section of the pulse risetime. Thus the pulses generated are a small fraction of the incoming blanking pulse risetime. However, it is then necessary to wait for the pulse to continue to its maximum height, return to a low level and rise again before the pulse can be repeated. This is a severe limitation in terms of both the jitter in the pulse and the amount of signal which can be recovered.
It is an object of this invention to provide improved means for generating a pulsed electron beam, in particular a pulsed electron beam containing very fast pulses at a high repetition rate enabling, for example, the investigation of operating ICs within which very high speed signals, possibly of many GHz bandwidth, are generated.